1. Field of the Invention
This invention relates to techniques for manufacturing products by inserting one or more punch(es) into one or more elongated recess(es) formed within a die so that a malleable material, such as a metal plate, is deformed into a shape conforming to the shape of the recess(es) in the die.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 3-264116 discloses a method for press bending a metal plate. Referring to FIG. 11(A), an example of such a known method is depicted, in which the punch 2 has been fully inserted into the die 4. Such a press 1 has previously been used to manufacture bumpers for automobiles. In this example, press 1 has been adapted for folding both ends of a steel plate W at right angles to a central portion of the steel plate W to thereby form a product generally having a U-shape cross-section. The press 1 includes the die 4 fixed at a predetermined position and the vertically movable punch 2.
The punch 2 extends in a direction perpendicular to the cross-section shown in FIG. 11(A). The punch 2 is bent along the lengthwise axis and therefore has a non-uniform radius of curvature along the length of the punch. FIGS. 11(B) and 11(C) each schematically show the shape of a product deformed by a punch 2 that is bent along the length of the punch. The punch 2 thus includes a generally linearly extending portion having a large or infinite bend radius and another sharply bent portion having a small bend radius. The die 4 also has an elongated recess 4m extending lengthwise that corresponds to the shape of the punch 2. The recess 4m is formed into a shape corresponding to the shape of the punch 2, so that the punch 2 can be inserted into the recess 4m.
The die 4 has a chamfered face 4f an opening edge thereof or along the border of an upper face 4u of the die 4 and the recess 4m. And most significantly, the chamfered face 4f has a uniform shape in cross section along the length of the border.
In the press bending operation, the flat plate W is placed on the upper face 4u of the die 4, and the punch 2 is then forced downward. As a result, the punch 2 is inserted into the recess 4m of the die 4 with the plate W between the recess 4m and punch 2. In the process of the punch 2 being fully inserted into the die 4, the plate W is deformed into a shape conforming to that of the punch 2 and the recess 4m. Thus, both ends of the plate W are folded approximately at right angles to the central portion of the plate W.
According to this known press-bending technique, the entire plate W is deformed at approximately the same time. For example, as shown in FIGS. 11(B) and 11(C), a generally linearly extending portion or region F (linear region) of the product is deformed approximately in synchronism with a bent region thereof. Throughout this specification, the reference symbol "OU" designates an outer bent region and the reference symbol "IN" designates an inner bent region.
In the linear region F, the length of the material that will ultimately constitute each sidewall of the final product does not change as a result of the press bending operation, as shown in FIGS. 11(B) and 11(C). Thus, the material having length c1 before the press bending operation has the same length c1 after the press bending operation.
On the other hand, in the inner bent region IN, the material is stretched lengthwise during the deformation process to thereby form the sidewall, as shown in FIG. 11(B). More specifically, in the example of FIG. 11(B), the material that will later constitute the sidewall has a length b2 before being deformed. After this material is deformed to create the sidewall, it has a length b1 in the final product, in which the length b1 is greater than the original length b2. Thus, the material in the inner bent regions IN is stretched or expanded from its original state. As a result, this stretched material is subjected to a tensile stress.
In the outer bent region OU an opposite effect is occurring at the same time. In this case, the material in the plate having length a2 before deformation is compressed to form the second sidewall. As shown in the example of FIG.11(C), this material has a length a1 after deformation, in which the length a1 is less than the original length a2. As a result, this compressed material is subjected to a compressive stress.
In this known technique, the entire plate W is deformed approximately at the same time without consideration to the influences of the expansion and compression of the material that will ultimately form the side walls. However, the expansion and compression of the malleable material directly affects the shape of the product. In particular, as shown in FIG.10(C), wrinkles J may form in the outer bent region OU as the result of the compression of the material. Further, the sidewalls in the inner bent region IN may have a lower height than the sidewalls in the linearly extending regions, because the material in region IN is stretched lengthwise during the deformation process.